Jet exhaust noise results from the turbulent mixing of exhaust gases with the atmosphere. The noise is influenced by the shearing action caused by the relative speed and temperature between the ambient and exhaust airflows. Jet exhaust noise may be reduced if the relative temperature and velocity is reduced. Exhaust noise can be especially troublesome for supersonic airplanes because current designs employ turbojets or low bypass ratio turbofans which require high velocity exhaust to produce the required thrust during takeoff.
Ejector nozzles are currently used extensively in pure jet and low bypass ratio engines as noise suppression devices. Generally, ambient air is introduced (i.e., aspirated) into a nozzle duct through auxiliary inlets, called ejectors. The ambient air is mixed with the hot engine exhaust, thereby reducing the overall velocity and temperature of the engine exhaust before it exits the nozzle. Mixing components are occasionally used in conjunction with the ejectors in the pure jet and low bypass ratio applications in order to more thoroughly mix ambient air with exhaust gas.
Turbojet and low bypass ratio engines require high aspiration levels (i.e., generally greater than about 60% of the engine exhaust) in order to provide adequate levels of noise suppression. High aspiration levels require the ability to significantly vary nozzle geometry. This requires ejectors capable of assuming a wide range of positions. In addition, the ejectors and mixing components must be capable of being selectively removed from the nozzle duct airflow path in order to transition the nozzle to an acceptable performance configuration when noise suppression is not required. The combination of these requirements often results in nozzle designs that are heavy, complex, and have low performance characteristics.
In contrast to the pure jet and low bypass ratio engines, moderately high bypass ratio engines (i.e., generally in the range of 0.6 to 1.0) usually create less jet exhaust noise to begin with due to their ability to produce thrust with lower average exhaust velocities. Noise reduction for moderately high bypass ratio engines generally consists of using a common or integrated exhaust nozzle that partially mixes the bypass and primary exhaust gases prior to their ejection into the atmosphere.
It is known to use ejector nozzles to improve nozzle performance in specific flight conditions. In particular, adding ambient air around the periphery of the exhaust gases of a bypass engine reduces aerodynamic boattail drag at transonic conditions. This is done during transonic and supersonic flight conditions where noise suppression is of no concern. For low bypass ratio applications, it is also known to use ejector nozzles to reduce noise. This is not the case for moderately high bypass ratio applications. Where the aerodynamic performance benefit is not sought, such as during takeoff and landing approach, ambient air is not added in a moderately high bypass ratio engine. Ejectors (for mixing ambient with engine exhaust) have not been used in moderately high bypass ratio turbofan engines for noise reduction for various reasons, including the generally held belief that the benefits of such use would be too small to justify the weight, space, and system complexity expenses associated with adding low bypass ratio ambient air noise suppression ejectors to moderately high bypass ratio jet engines.
In contrast to this belief, the inventors have recently discovered, however, that an optimum noise suppression solution for some aircraft is to aspirate a relatively small amount of ambient air into the engine exhaust of moderately high bypass ratio turbofan engines powering the aircraft. This has resulted in a need for such a nozzle. The ideal nozzle should not significantly add weight, space, or complexity to the overall engine system. Further, the ideal nozzle should include mixing components to further increase the mixing of ambient air with exhaust gas in a way that does not adversely affect the engine's performance.